Interesting logic puzzles and brain teasers

[vette982]

I thought it would be cool to start a thread with a bunch of interesting logic puzzles, brain teasers, math problems, etc. I'll get the ball rolling:

Your pharmacist gives you 5 jars filled with pills. The pills contained in each jar have an identical appearance and size (i.e. if you mixed all of the pills you wouldn't be able to tell them apart).

4 of the 5 jars hold pills that weigh 10 grams each, and the remaining jar holds pills that weigh 9 grams each. However, you don't know which jar is the one holding the lightweight pills. Keep in mind that all of the pills in all jars are identical in appearance and size.

You are given a weighing scale that can only be used once. Using this scale (only once), how do you determine which jar is holding the 9 gram pills?

Note: assume the jars are quite large and that you have an unlimited quantity of pills from each jar.

Solution: http://www.mindcipher.com/puzzles/141

 

[leroyjenkens]

I'm not sure if this counts, because I would consider it using the scale more than once. But I would put 1 pill from each jar onto the scale, and then take them off one by one. When you take off a pill and the reading is an even number, you know you took off the 9 gram pill.

 

[Jonathan Scott]

Note that there's already a "Brain Teasers" subforum of "Fun, Photos and Games", so perhaps this subject belongs there:

https://www.physicsforums.com/forumdisplay.php?f=33

 

[Curious3141]

I thought it would be cool to start a thread with a bunch of interesting logic puzzles, brain teasers, math problems, etc. I'll get the ball rolling:
Solution: http://www.mindcipher.com/puzzles/141

This belongs in the brainteaser thread.

Anyway, the solution is quite trivial.

SPOILER (white text):

Label the jars 1 to 5. Take 9,8,7,6,5 pills from each jar respectively. The last digit of the weight will indicate which jar it is.

EDIT: just saw the solution you linked, and honestly, I think mine's neater (posted it there as a comment).

 

[Evo]

Thread moved.

 

[leroyjenkens]

EDIT: just saw the solution you linked, and honestly, I think mine's neater (posted it there as a comment).

Did you think of that? Because that's pretty genius.

 

[vette982]

Yeah that's a super clean solution!

 

[Curious3141]

Did you think of that? Because that's pretty genius.

Yes, I did think of it, and thanks, but no, I don't think it's anything special.